Expression of the Head GeneLox22-Otxin the LeechHelobdellaand the Origin of the Bilaterian Body Plan

Abstract

TheorthodenticleorthologueLox22-Otxwas isolated from an annelid worm, the leechHelobdella triserialis. In situhybridization reveals that embryonic expression ofLox22-OtxRNA is primarily restricted to an unsegmented head domain, including tissues in the foregut, surface ectoderm, and the head ganglion of the central nervous system. The patterns of head expression form concentric rings about the stomadeum and mark tissue domains that exhibit discrete behaviors during later morphogenesis and differentiation. Expression was also observed in one to two bilateral pairs of neurons in each segmental ganglion or neuromere of the body trunk. The largely head-specific expression ofLox22-Otxin this annelid species supports data from two other bilaterian phyla in suggesting the existence of a genetically defined head/trunk distinction. We suggest here that this head/trunk distinction is a synapomorphy of the Bilateria as a whole, and that it reflects the body plan of an early bilaterian ancestor. In addition, we discuss the possibility that the radial organization of gene expression and cell lineages in the leech's head domain may reflect the symmetry properties of a prebilaterian ancestor that had a radially symmetric body plan.